1. Field of Invention
The present invention relates to a method for forming a bump, a semiconductor device and a method for making the same, a circuit board, and an electronic device.
2. Description of Related Art
A method for forming a bump composed of a metal or the like on a pad of a semiconductor chip by electroless plating is known.
In electroless plating, the bump grows not only in the height direction but also in the width direction (isotropic growth). The width of the bumps may therefore exceed the width of the pads. Thus, it is difficult to form bumps onto pads with a narrow pitch.
It is an object of the present invention, for solving the above problem, to provide a method for readily forming a bump with a desired width, a semiconductor device and a method for making the same, a circuit board, and an electronic device.
(1) The method for forming a bump in accordance with the present invention includes:
forming an opening in an insulating film which exposes at least a part of a pad;
forming a bump connected to the pad;
forming a resist layer having a through hole which overlaps at least a portion of the pad; and
forming a metal layer which is connected to the portion of the pad which is exposed at the opening.
According to the present invention, for example, the through hole in the resist layer communicates with the opening formed in the insulating film. The metal layer can thereby be formed connected to the pad. Thus, the bump can be formed by a simplified step. When the metal layer is formed in the through hole in the resist layer, the bump can be formed so as to have a size corresponding to the size of the through hole, in other words, a desired width.
(2) In this method for forming the bump, the through hole may be formed so as not to protrude from the periphery of the pad.
According to this configuration, the metal layer can be formed so as not to protrude from the periphery of the pad. Thus, a bump can be formed on each of a plurality of pads arranged at a narrow pitch.
(3) In the method for forming the bump, the insulating film may be thicker at the periphery than in the center of the pad.
The semiconductor chip can be, therefore, surely protected by the thick insulating film. The thick portion of the insulating film may be formed of a plurality of layers.
(4) In the method for forming the bump, the through hole may be formed inside the periphery of the pad and at about the center of the pad, in which the insulating film is thinner.
The bump can be thereby formed without exposing the pad.
(5) In the method for forming the bump, the metal layer may include a first metal layer and a second metal layer formed on the first metal layer.
(6) In the method for forming the bump, the opening may be formed so as to be larger than the periphery of the through hole so that a region for forming the first metal layer and an exposed portion of the pad are formed, and the second metal layer may be formed on the pad so as to cover the exposed portion.
Since the exposed portion of the pad is covered with the second metal layer even if the opening is larger than the through hole, the pad is not exposed.
(7) In the method for forming the bump, the first metal layer may be formed in the through hole, the resist layer may be removed, and then the second metal layer may be formed so as to cover the first metal layer.
Oxidation of the surface of the first metal layer is thereby prevented.
(8) In the method for forming the bump, the first metal layer may be formed in the through hole, and then the second metal layer may be formed on the first metal layer without removing the resist layer.
When a material having high affinity with a solder is selected as the second metal layer, the solder can be provided substantially only on the upper face of the metal layer. That is, the solder can be prevented from spreading from the periphery of the metal layer, and the solder can be provided without short-circuiting between adjacent pads.
(9) In the method for forming the bump, the first metal layer may be formed so as to protrude from the through hole so that the first metal layer has a tip having a width which is larger than the width of the through hole.
The tip of the first metal layer is larger than the width of the through hole. Thus, the bump has a space for holding a portion of the solder. Accordingly, the solder can be provided without spread from the periphery of the metal layer, that is, without short-circuiting between the pads.
(10) In the method for forming the bump, the second metal layer may be formed so as to protrude from the through hole so that the second metal layer has a tip having a width which is larger than the width of the through hole.
The tip of the second metal layer is formed so as to have a width which is larger than the width of the through hole. Thus, the bump has a space for holding a portion of the solder. Accordingly, the solder can be prevented from spreading from the periphery of the metal layer, that is, without short-circuiting between adjacent pads.
(11) In the method for forming the bump, the first metal layer may be formed by electroless plating.
(12) In the method for forming the bump, the second metal layer may be formed by electroless plating.
(13) The method for forming the bump may further include the step of providing a solder on the metal layer.
(14) In the method for forming the bump, in the step of providing the solder, a resin layer may be provided at the periphery of the metal layer other than at least the upper face so that the solder is provided at a portion, exposed from the resin layer, of the metal layer.
Since the resin layer repels the solder, an optimum amount of solder is provided on the metal layer. When the solder is melted, spreading thereof from the periphery of the metal layer can be prevented. Thus, in a plurality of pads of a semiconductor chip, contact of the solder with an adjacent pad is prevented.
(15) In the method for forming the bump, the metal layer may be formed so as to be substantially flush with the resist layer, and the solder may be provided at a portion, exposed from the resist layer, of the metal layer.
Since the same resist is used for forming the metal layer and the solder, the step is simplified.
(16) In the method for forming the bump, the first metal layer may be formed so as to be lower than the resist layer, and the second metal layer may be provided by a printing process through the resist layer as a mask.
Since the printed mask is the resist layer, the second metal layer can be provided regardless of releasability of the mask. Since no additional printed mask is formed, the second metal layer can be formed via reduced production steps.
(17) In the method for forming the bump, a conductive film electrically connected to the first metal layer at the periphery of the through hole may be formed on the insulating material, the first metal layer may be formed so as to be lower than the resist layer, and the second metal layer may be provided by electroplating using the conductive film as an electrode.
According to this method, a variation in the composition of the second metal layer can be suppressed compared to the formation by electroless plating. A variation in the melting point of the second metal layer is thereby prevented.
(18) In the method for forming the bump, the first metal layer may include a nickel-containing material.
(19) In the method for forming the bump, the second metal layer may include a gold-containing material.
(20) In the method for forming the bump, the second metal layer may include a solder.
(21) In the method for forming the bump, the solder may include Sn, or Sn and at least one metal selected from Ag, Cu, Bi, and Zn.
(22) In the method for forming the bump, the second metal layer may include first and second Au layers, the first Au layer being formed on the first metal layer by immersion plating and the second Au layer being formed on the first Au layer by autocatalytic plating.
The Au layer having a large thickness can be thereby formed on the surface of the bump. When the Au layer is formed on the surface of the first metal layer, the Au layer can be directly connected to a lead, as in the case in which the entire bump is formed of Au.
(23) In the method for forming the bump, the second metal layer may include an Au layer and a Sn layer, the Au layer being formed on the first metal layer by immersion plating and the Sn layer being formed on the Au layer by autocatalytic plating.
(24) In the method for forming the bump, in the step of forming the Sn layer, the electroless tin plating solution may contain at least one of Cu and Ag so as to deposit Sn and at least one of Cu and Ag.
According to this configuration, the bump can be satisfactorily connected to a lead even if the lead includes a material other than gold.
(25) In a method for making a semiconductor device in accordance with the present invention, the metal layer is formed on the pad formed in a semiconductor chip by the above method for forming the bump.
(26) The method for making the semiconductor device further may include the step of electrically connecting the bump to a lead, the second metal layer in the bump and the lead thereby forming an eutectic crystal.
(27) A semiconductor device in accordance with the present invention is produced by the above method for making the semiconductor device.
(28) A semiconductor device in accordance with the present invention includes:
a semiconductor chip having a plurality of pads;
an insulating film formed on the semiconductor chip so that the insulating film covers at least an end of each of the pads such that the pads have regions which are not covered with the insulating film; and
a bump formed on each pad, the bump including a first metal layer formed in the region which is not covered with the insulating film so as not to contact the insulating film, and a second metal layer formed so as to cover the first metal layer and contact the insulating layer.
(29) A semiconductor device in accordance with the present invention includes:
a semiconductor chip having a plurality of pads;
an insulating film formed on the semiconductor chip so that the insulating film covers at least an end of each of the pads such that the pads have regions which are not covered with the insulating film; and
a bump formed on each pad, the bump being formed so that the end thereof lies on the insulating film and the bump covers the region, and the thickness of a portion of the insulating film that is not above the pad is larger than another portion of the insulating film that is disposed below the bump.
According to the present invention, the surface of the semiconductor chip is covered with a thick layer and the insulating layer is thin below the end of the bump. Since the surface of the semiconductor chip is covered with the thick layer, the moisture resistance of the semiconductor chip can be enhanced. Since the step difference by the insulating film below the end of the bump is reduced, reliability of the connection between the pad and bump is enhanced.
(30) A semiconductor device according to the present invention includes:
a semiconductor chip having a plurality of pads; and
a bump including a pillar shaped body connected to each of the pads and a tip connected to the body, the tip having a width which is larger than the width of the body, the bump having a space for holding a solder between a portion exceeding the width of the body at the tip and the body.
According to the present invention, the bump has a space for holding a portion of the solder. When the solder is melted on the bump, the solder does not spread toward the exterior of the metal layer, thereby preventing mutual short-circuiting between adjacent pads.
(31) A circuit board in accordance with the present invention mounts the above semiconductor device.
(32) An electronic device in accordance with the present invention includes the above-mentioned semiconductor device.